Balancing circuit for telephonic transmission

ABSTRACT

178,860. Ges. f³r Drahtlose Telegraphie. April 23, 1921, [Convention date]. Telephone systems and apparatus.-In a substation bridge arrangement, singing, due to the passage of received currents through the transmitter when one of the bridge arms becomes unbalanced owing to a variation in the resistance of the line conductors, is avoided by a back-coupling between the unbalanced arm and the transmitter circuit. Such an arrangement is shown in Fig. 3, in which the line conductors 1 are initially balanced by the arm 2. The receiver 5 (which may be a coil coupled to the detector circuit of a receiving aerial) is connected between the junetion of the bridge arms 1, 2 and the mid-point of a subsidiary winding 6 in parallel with the winding 3 coupled to the transmitter 4 (which may be a thermionic generator associated with a transmitting aerial). One side of the winding 6 is coupled to the transmitter circuit through a winding 7 and a three-electrode amplifying valve 8. When the balance in the two halves of the winding 6 is upset owing to a change in the resistance of the line 1, received currents cause currents to flow in the windings 6 and 3, but the transmitter 4 is unaffected since a neutralizing current is induced in the transmitter circuit through the back-coupling 7. The valve 8 is arranged to have a limiting action in order to prevent the back-coupling 7 from interfering with transmission. A modification is described in which the valve 8 is omitted, the winding 7 being connected in series in the transmitter circuit. The invention is applicable to both high and low frequency telephony along conductors.

. LSIA@ A. MEISSNER BALANCING CIRCUIT FOR TELEPHONIC TRANSMISSION mec., 9, 92@

Filed May 5, 1922 rag, 2.

ALEXANDER MEJSSNE@ Patented 9, 1924..

UNITED STATESy PAT ALEXaNDEn nnrssNEn, or BERLIN, GERMANY, Assrervon 'ro ensnmscniarfr una nnsn'rnosn TELEGRAPHIE M. ze. n. namrscnns, or naamw, emmener, n 'enn- IPDRATION OF GERMANY.

BALANCING CIRCUIT FOR TELEPHGNIC TRANSMISSION.

Application led May 3,

'To all whom t may concern, Be it known that I, ALEXANDER Malss- NER, a citizen of Germany, residing at Berlin, Germany, have invented certain new and useful Improvements in Balancing Circuits for Telephonie Transmission, of which the following is a specificationl This invention relates to a system of signalling, more particularly telephony, using both land lines and Wireless. In systems of this kind, a central station is equipped with radio receiving and `sending apparatus and provision is made for connecting this apparatus to the subscribers land lines connectin with the central station.

telephonic transmission through wires with the aid of high frequency currents and `wireless telephony, a balancing circuit or Wheatsone bridge arrangement is employed which comprises the transmission line, an

artificial line made up of resistance and tuning elements, a transformer associated with the sender and a circuit which is branched l off fromfthe middle of the one coil of the transformer to the joinder of the transmissionline-arm and the artificial line arm and -is designed to receive the current from the distant station. Similar circuitv arrangements are also used for terminal or intereo mediate amplifying bridges in ordinary telephonic transmission over wires. If, in such arrangements, the artificial line is properly 4balanced with respect .to the real or transmission line, a reaction of the receiving circuit on the sender is obviated, and the production of howling noises inthe installation is revented.

e invention will be explained with reference to the accompanying drawings in which- Fig. 1 shows a simple bridge arrangement of the well known type, Fig. 2 shows-one feature of an arrangement according to the invention,

Fig. 3 shows a practically operative arrangement according to the invention; and Flg. t illustrates the regions of the' characteristie current curve of a thermionic valve that are used in receiving and sending. ln the ordinary bridge arrangement shown in Fig. 1, the transmission line 1 forms one arm of the bridge the second arm 2 of which consists of an artlfcial line. The other 'two 1922. serial No. seance.

arms are constituted by one of the windings of a transformer 3 from whose middle point a circuit containing receiving elements 5 is branched o. The secondv winding of the transformer 3 is connected with a sender 4.

The sender 4 and receiver V5 may be adapted .l

ing as the balancing is more or less imperfeet, and hence humming sounds or howling will be produced in the system.

4In Fi 1 the receiver 5 may be of the o rdinary etector type, and the sender 4 may comprise a thermionic valve, although it is indifferent whether a machine generator, an electric arc or a thermionic. valve is employed. .The transmission line 1 is an ordinary line leadin to a subscriber. The pur: pose of the bri ge arrangement is to prevent) the speech currents that proceed from the receiving device from affectin or creeping into the high frequency sen er. This object will be accomplished whenever the brldge is perfectly balanced. But if this is not the case due, for instance, to a defect in the line or to a different subscribers a paratus having been connected su it may appen that the resistance W1 of t e one arm of the bridge becomes smaller than the resistance W2 of the artificial line 2. The current J1 will then be greater than the currentJ", and the dierential current will then give rise to a flux in the transformer 3. If, under ysuch conditions, a vocal sound or" a letter such as a now arrives at the receiver 5, this letter a will be simultaneously sent outby the sender 4 and received at the corresponding distant station in which, of course, the bridge will also never be perfectly balanced. The receiver at the distant station will pass on the a to the sender at that station, which will eend it back to the esv lthe switching on of the system may be sufficient to start the vibrations, which will then be amplified to such a degree that the whole system will start humming.

This great drawback of all'balancing circuit arrangements is removed by the present invention. The arrangement according to this invention is such that whenever a flux due to imperfect balancing is translated from the receiving circuit to, or caused to arise in, the transformer, the efect of this flux is made to act on the sending circuit or its thermionic valve in such a manner that all reactions of the receiving circuit on the sender are compensated or neutralized.

IOne feature of the invention is exemplified vin Fig. 2. The circuit arrangement in this ligure differs from those of Fig. 1 in the first place in the fact that the receiving circuit is not directly connected to the sender transformer 3 but to a separate coil 6 which forms two arms of the Wheatstone bridge arrangement. One of the windings of the transformer 3 is connected in parallel to the coil 6. If `a flux is set up in the coil 6 due to lan unbalancing caused by variation of the resistanceof the line, a part of the current owing in the receiving circuit will act through the abovementioned winding of the transformer 3 on thel sending circuit 4.

In accordance with the invention, a second winding 7 is placed on the core of the coil 6. The potential induced in winding 7 1s always proportional to the degree of unbalance of the bridge arrangement, 1t being understood that the core of the coil 6 extends on both sides of .the point of connection of the receiver 5 to the coil, so that the currents J1 and J2 tend to neutralize' each other as to their effect on the .coil 6 and the winding 7. Since the winding 7 is connected in series with the secondary winding of the sending transformer 3,

it will act to compensate or neutralize whatever potential may have been directly produced in the secondar of the transforner 3 by currents int e receiving circui In the circuit arrangement shown in Fig. 3, the neutralizing or compensating voltage induced in the winding 7 is made to act on the sender 4 through a thermionic valve 8 that acts asa limiting device. The input circuit of the valve 8 is connected to winding 7 while the output circuit is connected to the primary of coupling 9. The secondary of coupling 9 is inv series with the secondary of transformer 3 andthe sender. The purpose of this limiting valve is to prevent the voice currents' sent over the line 1, which, of course, must be able to act upon the transformer 3, from also being compensated, as this would prevent these voice currents from being passed on. The heating current and anode voltage of the limiting valve 8are made such that the voltage impressed on the grid of valve `8 dueto the receiving circuit and reproduced in the sending circuit correspond to the region lying between the points a and t of the characteristic curve, shown by Fig. 4.. The characteristic curve shownin Fig. 4 is the usual curve in which the abscissae represent the grid voltage and the ordinates represent plate current. p,

In this case the valve 8 acts as an ordinary amplifier with no limitin action. The voltages induced in the sen er circuit by means of coupling 9 and transformer 3 are equal and opposite and hence the effect of the unbalance of the lVheatstone bridge on the sender is eliminated.

The voltage of the voice currents coming from the line 1 is usually from 10 to 20 times as great as the voltage of the currents coming from the receiver .and hence the voltage impressed on the grid of valve 8 due to these currents covers a much wider n range, as denoted for example, by the pointsy c and d. In this case, the valve .8' g1ves a limiting action and the voltages induced in the sender circuit by means ofy coupling 9 and `transformer 3 are not equalbut the voltage induced by means of transformer 3 predominates. Therefore, although the effects on the sender due to the receiving circuit are balanced, no neutralization of the voice currents from the line 1 takes place. l

The aforedescribed separation ofthe trans- 'former 3 from the balancing coil 6 in accordance with Figs. 2 and 3 makes it easier in many respects to obtain the desired balance. The currents due to imperfect balancing devide, and liow part through the transformer 3, and A part through the line 1, the greater part fiowing through the line, becausev it has a much smaller resistance than the transformer. The. arrangement of Fig. 3 therefore reduces the lelfect that a change of resistance of the line has on the sender. ,The sending back or return of energy to the sender circuit is also facilitated, because the t-ransformer 3 need not be altered. The coil 7 can be coupled with a separate balancing coil more easily than with a transformer.

intacto impedance of transmission line and artificial:

line.

2. In a balancingvcircuit arrangement for telephonie transmission. a. bridge circuit comprising a transmission line constituting one of the arms of the bridge, an artificial line constituting a second arm ot the bridge, a coil constituting the other two arms vof the bridge, and a receiver Vconnected across the junction of the transmission line and the artificial line, on the one hand, and an =intermediate point of the coil, on the other hand, a sender coupled to said coil, and

. means including a circuit'coupled with said coil for neutralizing the current impressed upon the sender by the bridge circuit due to unbalance in said bridge caused by unequal impedance of transmission line and artificialline.

3. In a balancing circuit' arrangement-for 'telephonic/transmission, a` bridge circuit comprising a transmission line, andan artilicial Aline constituting two arms of the bridgecircuit, and a coil connected across the terminals of the transmission line and the artificial line and constituting the other two arms 'of the bridge circuit, a receiverl connected across the bridge circuit betweenv the junction of the transmission line and the artificial line and a point intermediate between the ends of said coil. a winding connected in parallel with said coil, a sender coupledto the winding, and means, including a circuit coupled with said coil for neutralizing in the, sender current transmitted thereto by the winding due to unbalance in said bridge circuit arising from inequality of impedance of the transmission line and the artificial line.

4. In a balancing circuit arrangement for telephonie transmissin, a bridge circuit comprising' atransmission line constituting one of the arms of' the bridge circuit, an artificial line constituting a secondxarm of the bridge circuit, and a coil having an iron core and constituting the two other arms o the bridge circuit, a receiver 'connected across the bridge `circuit between thejunction of the transmission line and the artificial line and a point intermediate between the ends of said coil, a winding connected in parallel with said coil, a sender* coupled to said winding, and means lincluding a winding surrounding the core of said coil for neutralizing in the sendencurrent transmitted to the sender bythe first mentioned winding due to unblance in' said bridge caused by unequalimpedanoe of said trans# mission line and said artificial line.

5. In a balancing circuit arrangement for telephonie transmission, a bridge circuit comprising a transmission line constituting one of the arms of the bridge circuit, an

- artificialline constituting a seconda-rm of the bridge circuit, and a coil constituting the two other arms of the bridge circuit, a receiver connected across the bridge circuit between the junction of the transmission line and the artificial line and a point intermediate between, the ends of said coil, a Winding connected in parallel with said coil, a sender coupled to said winding, means for neutralizing in the sender current transmitted to the sender by the winding due to inequality of impedance of the transmission `line and the artificial line, said means comprising a three-electrode vacuum tube having its grid circuit coupled with said coil and its plate circuit coupled with the sender.

6. In a balancing circuit arrangement for telephonie transmission, a bridge circuit comprising a transmission line constituting one of the. arms of the bridge circuit, an artificial line constituting a'second arm of the bridge circ-uit, and a coil constituting the two other arms of the bridge circuit, areceiver "connected across the bridge circuit between the junction of the transmission line and the artificial line and a point intermediate between the ends vot said coil, a winding connected in parallel with said coil, a sender coupled vto said winding, and means for neu-k and thus -to` produce acurrent' of thel strength required for neutralization in the sender of the current transmitted to the sender from the receiver by said winding,

'and alsobeing arranged'to act with respect to the currents received from the 'transmission line as a limiting tube so thatthe currents from the transmission line arevnot neutralized. f i l 7. In a balancing circuit arrangement for telephoniel transmission, a bridge circuit comprisinga transmission line constituting one of the arms of the circuit, and' articia line constituting a second arm of the bridge circuit, and a coil constituting the two Other v'arms of the bridge clrcult, a recelver connectcd across the bridge circuit between the junction of the transmission line and the artificial line and a point intermediate between the ends of said coil, a Sender coupled to said coil, and means for neutralizing in the sender current transmitted to the sender by the bridge circuit due to unequal imedance of transmission line and artificial ine, said means comprising a thermionic vacuum tube coupled with the bridge circuit and with the sender.

8. `In a balancing circuit arrangement for telephonic transmission, a bridge circuit 'comprising a transmission line constituting one of the arms of the bridge circuit, an artificial line constituting a second arm of the bridge circuit, and a coil constituting the two other arms on the bridge circuit, a re-V ceiver connected acrossthe bridge circuit between the j unction ofthe transmission line and the artificial line and a point intermediate between the ends osaid coil, a Winding connected with said coil, a sender cou-v pled. to said winding, and means for neutralizing in thesender current transmitted to the sender by said winding due to unbalance of the bridge circuit caused by unequal impedance of transmission line and artificial line, said means comprising a three-electrode vacuum tube having its grid circuit coupled with the bridge circuit and its plate circuit coupled with the sender, and said vacuum tube being arranged to act with respect to the currents received from the receiver due to -unbalance of the bridge circuit as an amplifier and thus to roduce a current of the strength required or neutralization in the sender of the current transmitted to the sender from the receiver by said winding, and also being arranged to act with respect to the currents received from the transmission line as a limit-ing tube, so that the currents from the transmission line are not neutralized.

In witness whereof I hereunto set 'my hand in the presence of two Witnesses.

ERNST WILKE. 

